The increase in osmolarity from outer to inner medullary interstitium is maintained due to :
(i) Close proximity between Henle's loop and vasa recta
(ii) Counter current mechanism
(iii) Selective secretion of $\mathrm{HC}{{\mathrm{O}}_3}^{-}$ and hydrogen ions in PCT
(iv) Higher blood pressure in glomerular capillaries

The proximity between the Henle’s loop and vasa recta, as well as the counter current in them help in maintaining an increasing osmolarity towards the inner medullary interstitium.
PCT helps in selective secretion of ${\mathrm{H}}^{+}$, ammonia and ${\mathrm{K}}^{+}$ ions and reabsorption of $\mathrm{HC}{{\mathrm{O}}_3}^{-}$from it. It does not have my role in maintaining concentration gradient in the medulla.
Blood pressure in glomerular capillaries is responsible for glomerular filtration and not for counter current mechanism.